'Whale' Swam in Choppy Waters

By

Gregory Zuckerman And

Dan Fitzpatrick

Updated June 19, 2012 11:50 a.m. ET

J.P. Morgan Chase & Co. trader Bruno Michel Iksil at times resisted sharing some details of his positions with superiors, while trading executive Achilles Macris had a history of clashing with co-workers, according to current and former colleagues.

Mr. Iksil, a Frenchman known as "the London whale" for his outsize positions, and the Greek-born Mr. Macris are at the center of at least $2 billion of losses at the nation's...